1. Field of the Invention
The present invention relates to a large scale, non-chromatographic purification of chromogenic cryptahemispherands or lithium cryptahemispheraplexes which incorporate diaza-12-crown-4, diaza-14-crown-4 and diaza-15-crown-5 subunits and which are highly selective in binding of sodium ion in aqueous media.
2. Description of the Prior Art
In two review articles, Takagi, et al., "Crown Compounds as Alkali and Alkaline Earth Metal Ion Selective Chromogenic Reagents" Top. Curr. Chem. 121, 1984, pp. 39-65, and Lohr, et al., "Chromoand Fluoroionophores. A New Class of Dye Reagents" Acc. Chem. Res. 18, 1985, pp. 65-72, describe a great variety of chromogenic compounds based on macrocyclic polyethers (corands) which selectively bind metal ions producing a spectral shift in the chromophore. In most reported cases, chromatographic separation was the preferred means of purification of final chromgenic compounds. Much less frequently, when the chromogenic compound happened to be crystalline, recrystallization was used to obtain the pure product. Although chromatography is a powerful separation tool, its use becomes problematic in cases where chromogenic compounds bind ions very strongly. This issue is especially critical when purifying chromogenic compounds which show extremely high propensity toward sodium ion. Sodium ions are abundantly present in commonly used chromatographic materials such as silica gel and alumina, and can easily be scavenged by the chromoionophore during its attempted purification. It should be pointed out that such a danger of "poisoning" by complexed ionic species does not exist in case of chromogenic compounds derived from corands which are relatively weak binders. Additionally, the chromatographic purification is cumbersome and not cost-effective for large scale manufacture.
To date, only a few chromoionophores derived from much stronger macrocyclic binders such as spherands, hemispherands and cryptahemispherands have been reported. Cram, et al., "Host-Guest Complexation. 45. Highly Preorganized Chromogenic Spherands Indicator System Specific for Sodium and Lithium Ions" J. Am. Chem. Soc. 110, 1988, pp. 574, describe a chromogenic spherand of the general formula (I) below, which, due to its powerful binding, extracts sodium ions from Pyrex.RTM. and Kymax.RTM. volumetric flasks. ##STR2##
Chromogenic cryptahemispherands and their synthesis are described by Helgeson, et al., "Host-Guest Complexation. 50. Potassium and Sodium Ion-Selective Chromogenic Ionophores" J. Am. Chem. Soc. 111, 1989, pp. 6639-6650, and in U.S. Pat. No. 4,859,606 on which I am named as a co-patentee. These chromogenic cryptahemispherands are used in colorimetric assays of sodium and potassium in blood and other biological fluids. Such compounds can be presented as the following general formula (II) ##STR3## wherein: R, same or different, is hydrogen, lower alkyl, lower alkylidene, lower alkenyl, allyl, aryl or benzyl;
R', same or different, is lower alkyl, lower alkylidene, lower alkenyl, allyl, aryl or benzyl; PA1 R", same or different, is hydrogen, lower alkyl, lower alkylidene, lower alkenyl, allyl, aryl or benzyl; PA1 Z is halogen; PA1 Y is electron withdrawing group, e.g. CN, NO.sub.2, CF.sub.3, COOR; PA1 m is 1 to 3; PA1 n is 1 to 3; PA1 a is 1 to 3; PA1 b is 1 to 3; PA1 k is 1 to 3; PA1 l is 1 to 3; and PA1 x is 2 to 4. PA1 R', same or different, is lower alkyl, lower alkylidene, lower alkenyl, allyl, aryl or benzyl; PA1 R", same or different, is hydrogen, lower alkyl, lower alkylidene, lower alkenyl, allyl, aryl or benzyl; PA1 Z is halogen; PA1 Y is electron withdrawing group, e.g. CN, NO.sub.2, CF.sub.3, COOR; PA1 m is 1 to 3; PA1 n is 1 to 3; PA1 a is 1 to 3; PA1 b is 1 to 3; PA1 k is 1 to 2; PA1 l is 1 to 2; PA1 k and l cannot both be 2; and PA1 x is 2 to 4. PA1 if p=1, q=0, diazacorand 3 is diaza-14-crown-4. PA1 if p=0, q=1, diazacorand 3 is diaza-15-crown-5.
With reference to the above formula (II), in the chromogenic cryptahemispherand used to assay sodium, the compound is a lithium complex in which k and 1 can be either 1 or 2, but k and l cannot both be 2. As used hereinafter, chromogenic cryptahemispherand (1.1) refers to the compound of formula (II) wherein k and l are both 1. Chromogenic cryptahemispherand (2.1) refers to the compound of formula (II) wherein k is 1 or 2, and l is 1 or 2.
As mentioned earlier, chromogenic ionophores are usually purified by chromatography and chromatographic materials such as silica gel and alumina have high sodium content. It has been found that both chromogenic cryptahemispherand (1.1) and (2.1) have a high propensity toward sodium, thus any physical contact with materials containing this ion must be avoided. Due to stringent spectral requirements, sodium contamination of chromogenic cryptahemispherands (1.1) and (2.1) higher than 0.5% by weight disqualifies the compound from use in a sodium assay. The gel chromatographic purification method of the above U.S. Pat. No. 4,859,606 was found to be adequate when producing laboratory quantities of the purified chromogenic cryptahemispherands since the percentage yield of purified material was not critical. It was found, however, that such compounds could not be isolated in commercial quantities uncontaminated since during gel chromatography they scavenged alkali metal ions from the chromatographic materials and thus became tainted or poisoned. Attempted chromatographic purification of chromogenic cryptahemispherand (1.1) and (2.1) pursuant to the teachings of the aforenoted U.S. Pat. No. 4,859,606 afforded only a small fraction of the sodium-free material while most of the product was poisoned by sodium and therefore could not be used further in a sodium assay such as that disclosed and claimed in U.S. Pat. No. 4,859,606.
In U.S. Pat. No. 4,845,212, I describe a streamlined, simplified process for the preparation of compounds of formula (II), which process also includes purification by gel chromatography. It has been found that the same problems with contamination of the final compound arose in this process as well.
Elemental analysis of different types of silica gel and alumina which are commonly used chromatographic materials revealed the presence of abundant quantities of sodium ion (e.g. silica gel from Merck & Co., Inc. was assayed at 6.42 g Na.sup.+ /kg; silica gel from J. T. Baker was assayed at 6.49 g Na.sup.+ /kg) which could not be removed from the chromatographic material by an easy and economical way. Besides, even if this removal were successful, chromatographic purification would be costly and impractical for large scale commercial production of the chromogenic cryptahemispherands.
It is thus desirable to develop a purification procedure for such compounds which are susceptible to deactivation by sodium ion, as well as one which is highly cost effective as opposed to gel chromatography.
I have discovered that it is possible to circumvent the problem of sodium "poisoning" of the chromogenic cryptahemispherands by applying a non-chromatographic purification procedure. The procedure exploits differences between solubilities of the reaction mixture components. Gradual removal of unreacted organic components, inorganic salts and polymeric material leads to a pure, uncontaminated product. This approach proved to be the only practical way of producing sodium-free chromogenic cryptahemispherands (1.1) and (2.1), which exhibit extraordinary propensity toward the omnipresent sodium ion.